Clinical and Epidemiological Studies

Abstract

Assessment of the effects on humans of exposures to hazardous chemicals relies heavily upon the results of clinical and epidemiological studies. The great advantage of such studies over animal investigations is that they provide direct evidence of the toxic actions of chemical agents in humans. Human studies are, however, difficult to conduct properly, and frequently they are difficult for policy makers to interpret (Horwitz and Feinstein, 1979; Susser, 1977). A major source of this difficulty is that almost inevitably clinical and epidemiological studies must be conducted after the occurrence of exposure. Except in rare instances, ethical constraints preclude the prospective exposure of human subjects to toxic agents (Duncan et al., 1977; U.S. Department of Health, Education, and Welfare, 1978; Rothman, 1976). Thus, the typical study of the toxicity of chemical agents in humans is observational in nature and not experimental. An additional difficulty is posed here by the complexity of the causal relationships that typically exist between toxic exposures and human disease. Not only do individual humans vary greatly in their exposure to toxicants, but they vary also in their capacity for response as well as in their exposure to factors such as alcohol and tobacco, which may modify greatly the nature or severity of their responses to toxic exposures.

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